Energy changes in reactions
Exothermic reaction: results in an increase in temp of the
surroundings; as it gives out heat energy.
Endothermic reaction: results in a decrease in temp of the
surroundings; as it takes in energy.
Activation energy (Ea): the minimum energy colliding particles
must have to react.( energy needed to start breaking bonds)
Enthalpy(H)
The reaction in which the changes are happening is known as
the system. Everything else is known as the surroundings.
Enthalpy is the energy content of a system at constant
pressure.
The energy content of the reactant (H reactant)
The energy content of the product (H product)
= H products – H reactant
Exothermic reactions
The products have lower energy than the reactants. So
negative.
H is
H is –ve because some energy left the system [ into the
surrounding].
Since energy is given out into the surroundings the surrounding
temp. rises.
Endothermic reactions
The products have more energy than the reactants. so
positive.
is +ve because energy was absorbed into the system
[from the surroundings].
Since energy is absorbed in from the surroundings the
surrounding temp decreases.
is
Bonds
Bonds are forces of attraction between atoms/ions.
Breaking these bonds requires energy; to pull the atoms apart.
This energy is taken in from the surroundings. So [ temp of
surroundings decreases] it's an endothermic process.
Making bonds releases energy. So [ temp of surroundings
increases] it's an exothermic process.
Bond energy is the amount of energy ( in KJ) associated with
breaking/making 1 mole of bonds.
Energy difference
energy required to _ energy released when
break bonds
bonds are made
The change in energy going from reactants to product is called
change in enthalpy(
) or heat of reaction.
Exothermic: energy absorbed to break bonds is less than the
energy released when new bonds are formed.
is negative.
Endothermic: energy absorbed to break bonds is more than the
energy released when new bonds are formed.
is positive.
Calculations
Ex)
Measuring enthalpy change for
reactions experimentally
• Measuring enthalpies of combustion
• Measuring enthalpies involving solutions
Measuring enthalpies of combustion using calorimetry
Plan an experiment to find out which fuel ethanol or propane
produces more heat:
• Put 100cm into the calorimeter and close it.
• measure temp of water
• weigh the spirt burner containing ethanol
• light the burner. Stir the water with the thermometer while
continuously.
• put out the flame, when temp rise is about to reach 20c
• weigh burner imediately
• repeat previous steps using a burner containing propane.
• calculate temp rise per gram of fuel burnt for each fuel. And
compare.
When is this method used?
1- Comparing diffrent fuels to see which would give the most
heat to warm a known mass of water.
2- Compare the heat produced by the same mass of diffrent
fuels
How to make it a fair test?
Distance between wick and can is consistent
same volume of water used for all fuels
Measuring enthalpies involving solutions
Heat of nuetralization
Polystyrene cup is a good heat insulator. It is used as simple
calorimeter to measure the temperature rise of exothermic
reactions between solutions.
This is the procedure of the experiment
• The solutions are added into the cup and initial temp is
measured imediatly.
• The mixture is stirred well with a thermometer.
• Temp is constantly checked and maximum temperature is
recorded
Note: the equipment is used to measure heat energy given out
during the nuetralization reactions between acids and alkalis.
This energy change is known as Heat of nuetraliztion
This method is suitable in cases that include:
• Solid base reacting with acid
• Solid carbonate reacting with acid
• displacment reactions between metal ( Ch.9)
Fuels
A fuel is any substance we use to provide energy. We convert
the chemical energy in fuels to other forms of energy.
Hydrogen Fuel cell
A fuel cell is an electrochemical cell in which a fuel donates
electrons at one electrode and oxygen gais electrons at the
other.
At the anode:
At the cathode:
Overall reaction:
Oxygen + Hydrogen –> Water
The hydrogen-oxygen fuel cell produces energy by combing
both elements.Releasing only water and energy.
Summary of reaction in cell
• Air entering provides the oxygen
• Fuel (Hydrogen)
• Hyrogen loses electrons at the anode
• Oxygen reacts with the hydrogen
• Water is the only product at the cathode